The THRUSPECIES flag allows an actor to pass through other actors of the same species.
The MTHRUSPECIES flag allows a missile to pass through actors of the same species as whatever shot said missile.

The zip contains a test PK3 with an actor, "impling" that can pass through other doomimps and whose projectiles, of the class implingball, pass through as well. On the other hand, normal imps cannot pass through implings and normal doomimpballs cannot pass through implings as well.